Piniella gave the ball to Gallagher, who gave the Cubs (22-15) 4 1/3 innings in the team's 6-4 victory over Arizona to complete a three-game series sweep.
Gallagher allowed four runs on five hits and struck out six batters while walking three in a no-decision. Through four innings, he had allowed one run and two hits.
"It was a pretty decent start," Gallagher said. "I kept my team in the game."
Gallagher threw 83 pitches, 52 strikes. He did not find out he was starting until roughly an hour and 15 minutes before the game.
Cubs pitching coach Larry Rothschild "walked in and said, ‘You know you're starting, right,'" recalled Gallagher.
He said: "I felt I could have pitched a little better, but I'll take what I can for now."
Gallagher could make his next start when the Pittsburgh Pirates come to Chicago Friday. He has started his entire minor league career, but Chicago has often used him in mop-up appearances in the major league team's bullpen.
Cubs Assistant to the General Manager Gary Hughes said earlier this year that he would be surprised if Gallagher's future with the club is not as a starter.
"He's always done that and he's one of the guys that we have targeted as a future member of our rotation," said Hughes. "With him and Jeff Samardzija and Sean Marshall, they are really our best starting prospects at the higher levels."